Power steering systems assist a drive in turning at least two wheels of a vehicle. The driver engages a steering wheel and the power steering system is operably disposed along the mechanical linkage between the steering wheel and the turnable wheels of the vehicle. U.S. Pat. No. 5,070,956 describes a hydraulic power assist steering system having conventional relatively rotatable spool and valve body elements coupled to a vehicle steerable wheel and steering wheel for regulation of a hydraulic steering assist boost pressure, a torsion bar creating a mechanical centering torque between the spool and valve body elements, and an integral electromagnetic mechanism which defines an additional coupling of variable resilience between the spool and valve body elements for adjusting driver steering effort required to produce a given level of power assist.